There is a demand for capacitors for use in electronic instruments such as potable telephone and personal computer to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferably used because of its large capacitance per unit volume and good performance. In a tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body increased in surface area by pulverizing the tantalum powder. The former method of increasing the mass of the sintered body necessarily incurs enlargement of the capacitor shape and cannot satisfy the requirement for downsizing.
On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body is reduced or closed pores increase at the stage of sintering. Therefore, impregnation of the cathode agent in the later process becomes difficult.
As means for solving these problems, a capacitor using a sintered body of a material having a dielectric constant larger than that of tantalum is being studied. The material having a larger dielectric constant includes niobium. JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered element for capacitors, where an alloy mainly comprising tantalum, titanium, niobium, aluminum or the like is used as a valve-acting powder and the agglomerated powder of the alloy is molded under pressure into a niobium fine powder having a particle size of 2.0 μm or less. The fine powder is sintered, the molded and sintered body is cut into fine pieces, a lead part is joined thereto, and then those pieces are again sintered. However, JP-A-55-157226 neither discloses nor suggests the tantalum content or the amount of niobium powder and moreover, the properties of the capacitor manufactured using this powder are not disclosed at all.
U.S. Pat. No. 4,084,965 discloses a capacitor manufactured using a niobium powder of 5.1 μm obtained by hydrogenating a niobium ingot and pulverizing it. However, U.S. Pat. No. 4,084,965, neither discloses nor suggests the tantalum content and the amount of niobium powder.
Although niobium is deficient in the leakage current (hereinafter simply referred to as “LC value”), the present inventors have previously proposed that the LC value can be improved by nitriding a part of niobium (see, JP-A-10-142004). Increased reduction in the LC value can be attained, for example, by elevating the sintering temperature at the time of manufacturing the above-described niobium sintered body. However, if the sintering temperature is elevated, there arises a problem that the product of the capacitance (simply “C”) per mass of the sintered body manufactured and the chemical forming voltage (simply “V”) at the time of forming a dielectric material on the surface of the sintered body (hereinafter the product is simply referred to as “CV value”) becomes small.
The CV value is considered to be proportional to the surface area of the sintered body and, the surface area of the sintered body is estimated to depend on the specific surface area of the niobium powder with the same production conditions and molding conditions of niobium powder and with same sintering conditions in obtaining a sintered body. However, even if a niobium sintered body is manufactured from niobium powder using the same conditions in respective stages, the CV value of the manufactured niobium sintered body is not always the same but disadvantageously decreases.